This application is based on Japanese Patent Application No. 2000-365414, filed in Japan on Nov. 30, 2000, the contents of which are hereby incorporated by reference.
1. Field of the Invention
This invention relates to a switching apparatus for electric power filled with an electrically insulating gas. In particular, it relates to a gas-filled switching apparatus having an improved sealed container for housing switches or other components of the switching apparatus.
2. Description of the Related Art
FIG. 5 is a schematic side elevation of a cubicle-type gas insulated switching apparatus of the type to which the present invention relates. Switches such as disconnecting switches 201 and 302, a circuit breaker 301, and a grounding switch 303 are housed in sealed containers 2 and 3 provided inside a metal box 1. An electrically insulating gas fills the sealed containers 2 and 3. The insulating breakdown voltage of the insulating gas is roughly proportional to its pressure, so the gas is typically charged to a gauge pressure on the order of 0.03-0.2 MPa to enable the separation between components within the containers 2 and 3 to be decreased and thereby decrease the overall size of the switching apparatus.
Sulfur hexafluoride (SF6) is frequently used as the electrically insulating gas in the sealed containers 2 and 3. However, because sulfur hexafluoride has the capability of contributing to global warming, there are restrictions on its discharge into the atmosphere, so there is now an interest in the use of other electrically insulating gases for this purpose. One gas which can be used as a replacement for sulfur hexafluoride is dry air. However, dry air has a lower dielectric strength than sulfur hexafluoride gas, so it is necessary to pressurize dry air to a higher pressure than required for sulfur hexafluoride to obtain a comparable dielectric strength.
The sealed containers 2 and 3 are usually equipped with covers which can be detachably mounted on thick flanges surrounding openings at the ends of the containers. An airtight seal is formed between each cover and the corresponding flange by an annular elastic gasket which is received in a gasket groove formed in the front surface of the flange and clamped between the cover and the flange.
In some arrangements, a cover is secured to a flange by bolts. In other arrangements, a cover is secured to a flange by clips which fit over the cover and the flange and press them towards each other.
When bolts are used to secure a cover to a flange, the flange must be large enough to accommodate the bolts and bolt holes for receiving the bolts as well as the gasket groove, so there is the problem that the outer dimensions of the flange end up being large, which increases the overall size of the sealed container. Furthermore, as the dimensions of a flange increase, the distance from the support point of the flange to the location where a load is applied to the flange tends to be large, so the bending stresses at the support point become large, and the flange must be thick in order to resist the bending stresses.
When clips are used to secure a cover to a flange, it is unnecessary to provide bolt holes in the flange, so the width of the flange and the outer dimensions of the container as a whole can be decreased. Furthermore, the bending stresses applied to the flange can be lower than when bolts are used. However, the clips must exert a compressive force which can resist the gas pressure within the container to prevent the cover from becoming detached from the flange. And the gasket exerts an elastic force to the clips when compressed. Therefore, frictional forces generate between the clips, the cover and the flange by the compressive force by the clips and the elastic force by the gasket when the clips are installed on the cover and the flange. Due to these frictional forces, it is generally not possible to install or remove the clips by hand, and a special clip installation device is necessary for this purpose. When a cubicle-type gas-filled switching apparatus is installed in a confined space, there may be insufficient room for the clip installation device to operate, making installation and repair difficult.
The present invention provides a gas-filled switching apparatus having a sealed container with a cover which can be easily mounted on the container by hand to form a reliable airtight seal.
The present invention also provides a gas-filled switching apparatus having a sealed container which can withstand a high internal gas pressure.
The present invention additionally provides a gas-filled switching apparatus having a sealed container which can be small in size.
According to one form of the present invention, a gas-filled switching apparatus includes a sealed container containing an electrically insulating gas and a switch. The container includes a container body having a neck which surrounds an opening and a flange which extends outwards from the neck, a cover disposed on the flange and covering the opening, and a gasket disposed between the cover and the flange and forming a seal around the opening. An outer periphery of the flange lies inside an outer periphery of the container body as viewed in an axial direction of the container body. The container also includes a plurality of clamps detachably securing the cover to the flange. Each clamp receives a peripheral edge of the cover and the flange and includes an adjustable pressing member exerting a clamping force on the cover and the flange.
In preferred embodiments, the pressing member is a set screw. Preferably the set screw has a line of action passing through the gasket.
In preferred embodiments, the clamp has a generally U-shaped cross section with a pair of legs. One of the legs may flare away from the other leg at its end. The pressing member may be mounted on one of the legs.
In preferred embodiments, the gasket is received in a groove formed in the cover. The groove may be formed by a variety of methods, such as by press working or by machining.
In one preferred embodiment, a reinforcing plate is provided between the pressing member and either the cover or the flange so as to prevent abnormal deformation of the gasket by the clamping force of the pressing member.